The present invention relates to a demodulation system for a PSK modulation signal.
Conventionally, a demodulation system for a TDMA (time division multiple access) communication system must establish the synchronization condition within a short time, because the received TDMA signal has a plurality of asynchronous burst signals, and a reference signal must be generated for demodulating each burst. The transmitting side cannot send data until a synchronization is established with the receiving side. A preamble signal for establishing synchronization is attached preceeding the information data in a TDMA signal.
A preamble signal has, as shown in FIG. 1, carrier regeneration data P.sub.1, bit timing regeneration data P.sub.2 for regenerating a clock signal, and a unique word P.sub.3 for frame synchronization.
The length of a preamble depends upon the link quality. When the link quality is poor, and the received C/N (carrier to noise ratio) is low, a preamble must be long, as it takes a long time to establish the synchronization between a received signal and a reference carrier signal for demodulating the received signal.
By the way, in a satellite communication system, the carrier frequency of a received signal is, generally, different from that of the transmit frequency because of the incomplete frequency stability of earth stations, and/or the frequency conversion in a satellite. In case of a low speed digital communication system, which has a narrow bandwidth, the error of the received frequency due to the above reasons is even two or three times that of the bandwidth of the signal. When the TDMA signal has such a large frequency error of even two or three times that of the bandwidth, it takes a very long time for synchronization, and therefore, the preamble in that condition must be very long.
When the carrier to noise ratio C/N is low, and the frequency error is large, the length of a preamble must be long, as described above, and the length of a preamble must be equal to or longer than the data which follows the preamble.
The presence of a long preamble which does not send data decreases the efficiency of a TDMA communication system. In particular, the problem is serious when the data speed is low in a TDMA system.